The present invention generally relates to polishing of semiconductor wafers, and more particularly to chemical-mechanical planarization of semiconductor wafers with integrated circuits.
Semiconductor circuits are often fabricated by deposition of multiple layers of materials on a wafer of a high-purity semiconductor. Generally, the wafer and each layer should be a substantially uniformly planar surface. If the wafer, including the substrate and/or layers on the substrate, do not have substantially uniformly planar surfaces, difficulties may arise during fabrication of the semiconductor surface. For example, a conductive layer deposited over a nonplanar insulating layer may cause difficulties in conductive layer patterning and etching which may give rise to short circuits. In such a case, the conductive layer is not insulated electrically. More particularly, imaging fine-line patterns over the semiconductor wafer, as well as forming various components and interconnects, is often more difficult when a wafer surface is nonplanar.
Chemical-mechanical planarization (CMP) is sometimes performed to flatten the wafer and the successive layers. CMP involves application of a polishing pad, along with a slurry, to the surface of the wafer or the successive layers. Mechanical polishing occurs when the polishing pad is applied to the surface of the wafer or the successive layers. The slurry, often a chemically active abrasive, is used in conjunction with the polishing pad to remove wafer or deposited material.
A uniformly planar surface, however, is sometimes difficult to achieve using CMP. Nonuniformities in the removal rate of material, and thus in a resulting wafer surface, may occur for a variety of reasons. For example, flaws in the polishing pad may result in local nonuniformities in a wafer surface. In addition, there may be increased frictional contact between the polishing pad and the wafer at some areas due to the mounting of the polishing pad and wafer, or due to relative velocities of the polishing pad and wafer. The increased frictional contact may also result in a non-uniform surface. For many nonuniformities, however, the underlying cause may not be fully understood.
A nonplanar wafer surface is substantially uniformly planarized by a chemical mechanical planarization (CMP) process. The CMP process uses multiple sources of slurry delivered between a polishing pad and a substrate or wafer to achieve a planar wafer surface. The multiple slurry sources have differing compositions.
The slurry sources are delivered to different locations in between the substrate and the polishing pad. The delivery compositions of the different slurries of different locations depend upon the location of non-uniformities on the wafer. In an embodiment, the slurries bleed into each other during polish to form a radially-variable, slurry composition concentration gradient.
The polish removal rate due to the slurry composition is adjustable according to the location of the non-uniformities in the wafer surface, until substantially uniform planarization results.